In sheet-fed offset printing machines, the sheets to be printed are transported through the printing gap or nip formed between the rubber blanket cylinder and the counter-pressure cylinder by cylinders that are equipped with grippers. The transport of sheets between the printing stations also is carried out by cylinders or drums containing gripping devices. In order to guide the sheets onto the cylinders or drums without undesirable fluttering, sheet guiding apparatuses are provided at numerous locations in the printing machines. Such sheet guiding apparatuses guide the sheets onto the cylinder surfaces or metal guides by means of blasting air and/or suction air.
In order to ensure an uninterrupted printing process, it is important that the sheets being introduced into a printing gap, such as between a counter-pressure cylinder and a rubber blanket cylinder, lie on the cylinder in a precisely positioned fashion, and that this position is not changed while the sheets pass through the printing gap. A variety of sheet guiding apparatuses that operate by means of blasting or forced air have been developed for this purpose. In these devices, the sheet is pressed onto the cylindrical surface of the counter-pressure cylinder by means of the blasting air. However, a disadvantage of such sheet guiding apparatuses is that the components of the sheet guide, such as air directing tubes, cannot extend sufficiently far into the space between the counter-pressure cylinder and the rubber blanket cylinder for easy direction of the air. The jets of blasting air consequently must be precisely aligned in order to achieve the desired effect of fixing the sheet in place. It also is disadvantageous that the tendency for sheets to flutter usually increases with an increased pressure of the blasting air, in particular, when processing thin sheets.
Another problem arises if the rubber blanket cylinder is put down relative to the counter-pressure cylinder in a particular printing station, i.e., the sheet situated on the counter-pressure cylinder needs to be transported through the gap at the location without contacting the rubber blanket cylinder. Any contact between the sheet that was printed in preceding printing stations and the surface of the rubber blanket cylinder must be avoided because resulting smearing would lead to wasted sheets.
A blasting air device for fixing a sheet on a counter-pressure cylinder is known from DE 197 15 964 C1. In this device, the jets of blasting air are timed, i.e., the blasting air only acts upon the sheet while it passes through a respective section.
It also is known to fix the position of a sheet situated on a cylinder or another surface by means of electrostatic forces. In the apparatus shown in EP 0 737 572 B1, such an electrostatic device serves for holding sheets situated on a cylinder while they are printed by means of an auxiliary printing device (i.e. a laser printer or inkjet printer).
An electrostatic sheet holding device arranged in the region of the printing gap between a counter-pressure cylinder and a rubber blanket cylinder also is known from U.S. Pat. No. 3,174,748. That device includes a rod that extends over the entire format width. This means that the sheet passes through a gap formed between the rod-shaped electrode and the surface of the counter-pressure cylinder before it is introduced into the printing gap.
In electrostatic devices, the forces that hold the sheets in position are created by the electrically insulating sheet material in connection with an electric field and a charge polarization. In such case, an electric field is created on the surface of the sheet that lies on the counter-pressure cylinder, and an electric charge is generated that causes an electrostatic force between the sheet and the printing cylinder.
One disadvantage in such electrostatic sheet guiding apparatuses is that it is only possible to fix a sheet in position that already lies placidly on the cylinder. In addition, certain geometric conditions also have to be observed such that the arrangement of the electrode in the gap between the rubber blanket cylinder and the counter-pressure cylinder is restricted.